The present invention relates to a positioning control system which can be applied to various types of industrial robots or numerical control (NC) machine tools.
First, explanation will be made about a prior-art positioning control system with reference to FIG. 1. There is shown a block diagram of the prior art in FIG. 1 wherein a microcomputer 1 supplies data X indicative of movement amount of a robot (not shown) to a pulse distributor 2 whereby the robot is positioned. More specifically, a data request signal DR is sent cyclically from the pulse distributor 2 to the micro computer 1 so that each time the micro computer 1 receives the data request signal DR the micro computer 1 sends the data X to the pulse distributor 2. This causes the pulse distributor 2 to send to a deviation counter 3 a command pulse signal CP having the number of pulses corresponding to the data X. The counter 3 counts up in response to the command pulse signal CP and supplies the counted value to a digital/analog converter (which will be referred to as the D/A converter, hereinafter) 4. The D/A converter 4 in turn converts the received count value to a D.C. voltage which is applied as a drive voltage to a motor 5. On the other hand, on a shaft of the motor 5 is mounted an incremental encoder 6 for generating a feedback pulse signal fP of a frequency corresponding to the rotation amount of the motor 5. The feedback signal fP is applied as a count-down clock signal to the deviation counter 3, whereby the drive voltage to be applied to the motor 5 is dropped. Therefore, (1) when the command pulse signal CP is higher in frequency than the feedback pulse signal fP, the speed of the motor 5 is accelerated, (2) when the signal CP is equal to the signal fP with respect to frequency, the speed of the motor 5 is kept at a constant level, and (3) the signal CP is lower in frequency than the signal fP, the speed of the motor 5 is decelerated. In this way, the robot positioning operation can be achieved by controlling the value of the data X cyclically applied to the pulse distributor 2 to drive the motor 5 under control of the microcomputer 1.
However, in the case of the prior-art positioning control system of the type referred to above, it is necessary to return the robot to the initial start point (which is called an origin return operation) before power is initially supplied to the system, because the prior art uses the incremental encoder 6 and thus the robot position is detected in the form of a relative position with respect to an origin. More particularly, in the case of the prior art system, the origin return is effected in a software manner by entering an origin detection signal LS from a mechanical limit switch 7 as well as an origin pulse signal BP from the incremental encoder 6, into the microcomputer 1. The origin return operation of the prior art will be detailed by referring to FIG. 2 in which the horizontal (abscissa) line indicates the robot position with respect to time.
When the robot is moving from the left to the right in FIG. 2 at a speed of V.sub.max, the microcomputer 1 sends the data X of value .DELTA.X.sub.max to the pulse distributor 2 in response to the data request signal DR. If the robot is moved into the vicinity of the origin and the mechanical limit switch 7 provided in a robot passage in the origin vicinity is turned on, then the origin detection signal LS is switched to high logical level, so that the microcomputer 1 sends the data X of value .DELTA.X.sub.1 (whose target robot speed is V.sub.1 and .DELTA.X.sub.1 is less than .DELTA.X.sub.max) to the pulse distributor 2 in response to the data request signal DR. As a result, the frequency of the command pulse signal CP is decreased and the motor 5 is decelerated. Thereafter, when the robot speed reaches V.sub.1, the frequency of the command pulse signal CP becomes equal to that of the feedback pulse signal fP, whereby the robot is moved at a fixed speed from the left to the right in FIG. 2. The microcomputer 1, on the other hand, still monitors the original detection signal LS and the origin pulse signal BP in such a manner, that as soon as the robot has passed the limit switch 7 to change the origin detection signal LS to low logical level, the computer 1 will detect the first pulse or spike in the origin pulse signal BP appearing after the level transition of the signal LS to low level and will send the data X of value 0 (zero) to the pulse distributor 2 in response to the data request signal DR. When the value of the data X goes to zero, the distributor 2 stops the supply of the command pulse signal CP to the deviation counter 3, whereby the counter 3 is counted down by the feedback pulse signal fP. When the counter 3 is counted down to zero, the output voltage of the D/A converter 4 becomes zero and the robot stops.
However, such a prior-art positioning control system has a defect in that the accurate origin return operation can not be effected because a robot coasting distance L.sub.1 upon the origin return is long and not constant. Of course, it is possible to reduce the coasting distance L.sub.1 and variations in the distance L.sub.1 by lowering the robot moving speed V.sub.1 itself, but this results in that the time necessary for the robot origin return operation is made long.